1. Field of the Invention
This invention relates to optical media and more particularly relates to the depression depth made in optical media.
2. Description of the Related Art
An optical data storage medium such as a compact disc (“CD”), digital versatile disc (“DVD”), DVD read only memory (“DVD-ROM”), High Definition DVD-ROM (“HD-DVD-ROM”), writable DVD and HD-DVD media, Blu-Ray ROM, Blu-Ray writable media, and the like, stores digital data that is retrieved using a radiation beam such as the emission of a laser diode. FIG. 1 is a schematic diagram illustrating one embodiment of an optical data storage device 100 for the optical data storage medium 130. The device 100 includes an optical module 105, an arm 110, an optical head 115, a clamping spindle 120, a spindle motor 125, and the optical data storage medium 130.
The optical data storage medium 130 (herein referred to as “Disc”) is removably mounted on the clamping spindle 120. The spindle motor 125 rotates the Disc 130. The arm 110 positions the optical head 115 to retrieve data from the Disc 130. In one embodiment, the optical module 105 includes a voice coil motor that transports the arm 110 and the optical head 115 radially relative to the Disc 130. The combination of the rotation of the Disc 130 by the spindle motor 125 and radial movement of the optical head 115 may position the optical head 115 over any portion of the Disc 130 that is used for data storage.
FIG. 2 is a schematic diagram illustrating one embodiment of an optical path 200 of the optical data storage device 100 of FIG. 1. The optical head 115, arm 110, and optical module 105 of FIG. 1 may comprise optical path 200. The optics path 200 includes a holder 212, one or more lens 214, 222, 228, 234, 250, a mirror 216, an arm path 218, one or more optical detectors 220, 238, 240, a first beam splitter 224, a circularizer 226, a laser diode 230, a multiple data surface filter 232, a second beam splitter 244, a half-wave plate 242, a polarizing beam splitter 236, an astigmatic lens 246, a focus actuator motor 256, and a quad optical detector 248. Also depicted is the Disc 130 of FIG. 1 with a plurality of data surfaces 205 and a plurality of spacer layers 210.
The laser diode 230 may be a gallium-aluminum-arsenide diode laser that produces a primary radiation beam 252. In one embodiment the radiation beam 252 is in the range of 630 nm to 670 nm. In an alternate embodiment, the radiation beam 252 is in the range of 385 nm to 425 nm. The radiation beam 252 is collimated by the third lens 228 and is circularized by the circularizer 226 which may be a circularizing prism. The radiation beam 252 passes to the first beam splitter 224. A portion of the beam 252 is reflected by the first beam splitter 224 to the second lens 222 and the first optical detector 220. The first optical detector 220 monitors the power of radiation beam 252.
The rest of radiation beam 252 passes through the arm path 218 to the mirror 216. The arm path 218 may be a variable length optical path between the first beam splitter 224 that resides in the optical module 105 of FIG. 1 and the mirror 216 that may reside in the optical head 115 of FIG. 1. The beam 252 is reflected by the mirror 216 and passes through the first lens 214 and the multiple data surface aberration compensator 250 and is focused onto one of the data surfaces 205 of the Disc 130. As depicted, the radiation beam 252 is focused on the second data surface 205b. 
The first lens 214 is mounted in the holder 212. The position of holder 212 is adjusted relative to medium 12 by the focus actuator motor 256 which may be a voice coil motor. The focus actuator motor 256 may position the first lens 214 relative to the Disc 130 to focus the beam 252 on any one of the data surfaces 205.
A portion of the radiation beam 252 may be reflected at the data surface 205 as a reflected beam 258. The reflected beam 258 returns through the compensator 254 and the first lens 214 and is reflected by the mirror 216. At the first beam splitter 224, the reflected beam 258 is reflected through the multiple data surface filter 232. The reflected beam 258 passes through the multiple data surface filter 222 and passes to the second beam splitter 244.
At the second beam splitter 244 a first portion of the reflected beam 258 is directed to the astigmatic lens 246 and the quad optical detector 248. The quad optical detector 248 is divided into four equal sections. The quad optical detector 248 detects and provides focus and tracking information in response to the reflected beam 258. When the radiation beam 252 is focused on the data surface 205, the reflected beam 258 is focused on the quad optical detector 248 with a circular cross section with each of the sections of the quad optical detector 248 receiving substantially equal radiation.
If the radiation beam 252 is not focused on the data surface 205, the reflected beam 258 is focus on the quad optical detector 248 with an oval cross section. As a result, one or more quad optical detector 248 sections receive more radiation than other sections. The focus error of the radiation beam 252 is estimated from differences in radiation received by the sections, and the optical module 105 may correct the focus. For example, the focus actuator motor 256 may position the holder 212 and the first lens 214 to focus the radiation beam 252 in response to reflected beam 258 radiation pattern on the quad optical detector 248.
A second portion of the reflected beam 258 is directed from the second beam splitter 244 through the half-wave plate 242 to the polarizing beam splitter 236. The polarizing beam splitter 236 separates the reflected beam 258 into a first orthogonal polarized light component 260 and a second orthogonal polarized light component 262. The fifth lens 250 focuses the first orthogonal polarized light component 260 on the third optical detector 240 while the fourth lens 250 focuses the second orthogonal polarized light component 262 on the second optical detector 238. The second and third optical detectors 238, 240 detect the reflected beam 258 or absences of the reflected beam 258, (both referred to herein as “Detections”). The second and third optical detectors 238, 240 further provide detection signals in response to the Detections of the reflected beam 248.
FIG. 3 is a cut-away perspective drawing illustrating one embodiment of a portion of the DISC 130 of FIGS. 1 and 2. As depicted, the Disc 130 may be a dual-layer DVD-ROM media. In addition to DVD-ROM media, the Disc 130 could be HD-DVD-ROM media, or Blu-Ray ROM media. Alternately, FIG. 3 may depict the stamped headers of recordable data sectors in recordable optical media.
The Disc 130 includes one or more spacer layers 210 and one or more data surfaces 205. For illustrative purposes, the thickness of the data surfaces 205 and spacer layers 210 are not drawn to scale. In one embodiment, a spacer layer 210 such as the first spacer layer 210a may also be referred to as the substrate. Each spacer layer 210 is configured to transmit a radiation beam 252 such as the radiation beam 252 of FIG. 2. In addition, each spacer layer 210 has an index of refraction that is a physical property of the spacer layers 210.
The first data surface 205a may be coated with a semi-transparent/semi-reflective coating, such as gold. The radiation beam 252 may either be reflected off of the first data surface 205a or transmit through the first data surface 205a to the second data surface 205b. The second data surface 205b is typically highly reflective and coated with aluminum or another highly reflective coating. In one embodiment, for Discs 130 with three or more data surfaces 205, all data surfaces 205 are semi-transparent/semi-reflective except for the inner data surface 205 which is reflective.
A lens 214 such as the first lens 214 of FIG. 2 focuses a radiation beam 252 such as the radiation beam 252 of FIG. 2 on either the outer first data surface 205a or the inner second data surface 205b. For purposes of illustration, a first and second instance of the first lens 214a, 214b is depicted focusing a first and second radiation beam 252a, 252b on the first and second data surface 205a, 205b. However, a single radiation beam 252 is typically employed.
The first data surface 205a is physically separated from the second data surface 205b by a second spacer layer 210b. Each data surface 205 is depicted with one or more pits 305. The pits 305 are of a specified depth to cause light cancellation so that the pits look dark to the optical path 200. In one embodiment, the pits 305 are stamped into the data surface 205. The stamped data surface 205 may be physically connected to the spacer layer 210. A plurality of data surfaces 205 and spacer layers 210 may be physically connected to form the Disc 130.
In a certain embodiment, the pits 305 are stamped into a groove or an inverted groove of the data surface 205. Recordable media typically has such stamped grooves, and adjacent lands there between, where data is recorded by the user. Sector headers, marking the fixed block architecture of the recordable media, may be encoded with pits within the grooves and on the lands, as sector headers are intended to be read-only. The groove or inverted groove may be formed on the data surface 205 to aid in tracking and correction the focus of the radiation beam 252.
The first radiation beam 252a is focused on a fifth pit 305a of the first data surface 205a. The second radiation beam 252b is focused on a third pit 305c of the second data surface 205b. The depth of a pit 305 is selected such that when the radiation beam 252 is focused on the base of the pit 305, as is depicted for the third and fifth pits 305c, 305e, the radiation beam is reflected from the pit 305 with a phase that is substantially one hundred and eighty degrees out of phase from the radiation beam 252 entering the pit 305. The interference of the reflected beam 258 and the radiation beam 252 is detected by the second and third optical detector 238, 240, detecting the pit 305.
The depth d of the pit 305 has been determined as a function of the wavelength λ of the radiation beam 252 and the index of refraction n of the spacer layer 210, as illustrated in Equation 1.
                    d        =                  λ                      4            ⁢            n                                              Equation        ⁢                                  ⁢        1            
Because each spacer layer 210 may have a unique index of refraction, each data surface 205 may have a different pit 305 depth. For example, Equation 2 illustrates the relationship between the pit depth d1 for the first data surface 205a and the index of refraction n1, of the first spacer layer or substrate 210a, the pit depth d2 for the second data surface 205b and the index of refraction n2 of the second data surface n1, and the radiation beam 252 wavelength λ.
                                          d            1                    ⁢                      n            1                          =                                            d              2                        ⁢                          n              2                                =                      λ            4                                              Equation        ⁢                                  ⁢        2            
Unfortunately, providing for a unique pit depth for each data surface 205 increases the complexity of manufacturing the Discs 130 as multiple pit depths must be stamped. The increased complexity increases the cost of manufacturing Discs 130.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that calculate a common Disc 130 pit depth. Beneficially, such an apparatus, system, and method would reduce the manufacturing costs of Discs 130 by allowing each ROM data surface 205 to be stamped with pits 305 of a common pit depth, or each rewritable surface to be stamped with grooves of a common groove depth.